This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. xc2xa7119 and xc2xa7120 from an application for A COLD-START WAVELENGTH-DIVISION-MULTIPLEXED OPTICAL TRANSMISSION SYSTEM AND METHOD FOR GENERATING STANDARD WAVELENGTHS THEREFOR earlier filed in the Korean Industrial Property Office on May 7, 1997 and there duly assigned Serial No. 17558/1997.
1. Field of the Invention
The present invention concerns a cold-start wavelength-division-multiplexed (WDM) optical transmission system and method for generating standard wavelengths used therefor.
2. Description of the Related Art
The WDM system is used to multiplex a plurality of transmitter lasers on a single optical fiber, considerably increasing the transmission capacity per optical fiber even if the lasers operate at low speed. Hence, it enables the transmission capacity to be increased without replacing the embedded single-mode fiber with a distributed transition optical fiber. However, WDM systems require each transmitter laser to operate at a specific wavelength to the end of the systems"" lifetime. Proposals to standardize these wavelengths are currently being reviewed by the International Telecommunication Union (ITU). Therefore, all the lasers to be used in WDM systems will be operated at the standard frequencies recommended by the ITU.
At present, practical WDM system use distributed feedback (DFB) lasers as the existing optical transmission system since various types of tunable lasers have yet to show the required reliability, manufacturability and mode stability. However, DFB lasers have limited wavelength tunability and are difficult to manufacture to precise standardized wavelengths. Thus, DFB lasers must be xe2x80x9cwavelength-selectedxe2x80x9d for each channel, and then tuned by temperature to operate at the standardized wavelengths. Once the DFB laser is tuned either in the factory or in the field, it must maintain the wavelength over the expected system""s lifetime. However, because of aging, the wavelengths of some DFB lasers may drift up to a few tens of gigahertz even when their operating conditions such as bias current and temperature remain constant. This drift would result in the reduction of system margin and cause crosstalk. This may complicate the installation and maintenance of WDM systems. In particular, it could be disastrous if the wavelength of any laser must be changed in the field by unskilled technicians in order to restore the impaired transmission quality.
The following patents each disclose features in common with the present invention but do not teach or suggest the specifically recited cold-start wavelength division multiplexed optical transmission system and method of the present invention: U.S. Pat. No. 5,589,969 to Taga et al., entitled Wavelength Division Multiplexed Optical Fiber Transmission Equipment, U.S. Pat. No. 5,606,573 to Tsang, entitled Method And Apparatus For Control Of Lasing Wavelength In Distributed Feedback Lasers, U.S. Pat. No. 5,469,288 to Onaka et al., entitled Optical Filter, Method Of Controlling Transmission Wavelength Thereof, And Optical Receiver Using The Method, U.S. Pat. No. 5,557,439 to Alexander et al., entitled Expandable Wavelength Division Multiplexed Optical Communications Systems, U.S. Pat. No. 5,566,018 to Lee et al., entitled Apparatus For Adjusting Channel Width Of Multi-Channel Fiber Amplifier Light Source, U.S. Pat. No. 5,387,992 to Miyazaki et al., entitled Optical Frequency Multiplex Carrier Control System, U.S. Pat. No. 5,305,134 to Tsushima et al., entitled Optical Frequency Division Multiplexing Transmitter And Optical Frequency Division Multiplexing Transmission Apparatus, U.S. Pat. No. 4,635,246 to Taylor et al., entitled Frequency/Multiplex System Using Injection Locking Of Multiple Laser Diodes, U.S. Pat. No. 4,930,855 to Clark et al., entitled Wavelength Multiplexing Of Lasers, U.S. Pat. No. 4,813,756 to Frenkel et al., entitled Etalon Filters For Optical Channel Selection In Wavelength Division Multiplexed Fiber Systems, U.S. Pat. No. 5,119,454 to McMahon, entitled Bulk Optic Wavelength Division Multiplexer, U.S. Pat. No. 5,283,845 to Ip, entitled Multi-Port Tunable Fiber Optic Filter, and U.S. Pat. No. 5,438,579 to Eda et al., entitled Wavelength Stabilizing Apparatus.
It is an object of the present invention to provide a WDM optical transmission system which enables each laser to automatically find its operating wavelength and adjust its output power.
It is another object of the present invention to provide a WDM optical transmission system which controls the transmitter lasers to maintain the stable operation at the standardized frequencies to the end of the system""s lifetime.
It is still another object of the present invention to provide a WDM optical transmission system which prevents the reduction of the system margin resulting from the wavelength drift.
It is a further object of the present invention to provide a WDM optical transmission system which prevents the crosstalk resulting from the wavelength drift.
It is further another object of the present invention to provide a WDM optical transmission system which simplifies the installation and maintenance thereof.
It is further another object of the present invention to provide a WDM optical transmission system with a device for generating the standardized laser frequencies.
The inventive WDM system utilizes DFB lasers and passive optical demultiplexers. The DFB lasers are tested under normal operating conditions (e.g., biased at 40 mA at 25xc2x0 C.) and are wavelength-selected for each channel. The criteria for the wavelength selection are to have the laser operating within one half of the channel spacing from the standardized wavelength. However, these lasers are not necessarily tuned to the standardized wavelengths in the factory or in the field. Thus, for the cold start operation, each laser is able to automatically find its operating wavelength without any prior knowledge of its operating conditions for the standardized wavelength. In addition, this system may simultaneously adjust the output power of each laser to a desired value. Of course these lasers must operate at the standard frequencies to the end of the system""s lifetime.
According to one aspect of the present invention, a WDM optical transmission system comprises: a reference providing device for providing equally-spaced references at standardized frequencies, a plurality of DFB transmitter lasers for operating near the standardized frequencies for different channels, a passive optical multiplexer for multiplexing the outputs of the DFB lasers, a controller for controlling the DFB laser to automatically operate at the standardized frequencies to generate outputs of desired levels, and a passive optical demultiplexer for demultiplexing the WDM output of the passive optical multiplexer.
The reference providing device is achieved by employing a solid etalon filter. A part of the output of the passive multiplexer is delivered to the etalon filter to provide references at the standardized wavelengths. The remaining part is delivered to the demultiplexer. If the multiplexer has only one output, a 2xc3x972 star coupler is added to generate two outputs. The controller includes a photo-detector for converting the optical output signal of the etalon filter into an electrical signal, a phase-sensitive detector for detecting the first derivative signal from the etalon filter, and a control circuit for changing the operating temperature of the DFB lasers so as to conform their frequencies to the resonant frequencies of the etalon filter according to the first derivative signal obtained from the phase-sensitive detector.
If the output signal of the phase-sensitive detector is zero and has negative slope, the frequency of the laser is considered to conform to the corresponding resonant frequency of the etalon filter, since the phase-sensitive detector receives the first derivative signal of the etalon filter as the input according to the laser frequency. The controller controls the phases sensitive detector to generate a zero output with a negative slope. In addition, a current control circuit consisting of a proportional amplifier and an integrator is provided to precisely adjust the bias current of each laser by processing the first derivative signal, so that the laser frequency is locked to the desired resonant peak when the laser frequency approaches the corresponding resonant frequency.
The present invention will now be described more specifically with reference to the drawings attached only by way of example.